Michael William Zintl

Control of ion temperature anisotropy in a helicon plasma

Laser induced fluorescence measurements of the parallel and perpendicular ion temperatures in a helicon source indicate the existence of a substantial ion temperature anisotropy, . The magnitude of the ion temperature anisotropy depends linearly on the source magnetic field. The parallel ion temperature is independent of magnetic field strength while the perpendicular temperature increases linearly with increasing magnetic field. Bohm-like particle confinement is proposed as an explanation for the linear dependence on magnetic field of the perpendicular ion temperature. In the helicon mode, the ion temperature components are independent of RF driving frequency and power and show a trend towards isotropy at high neutral fill pressures.

Laboratory simulation of broadband ELF waves in the auroral ionosphere

Space-relevant observational signatures, such as frequency spectra, phase velocities, excitation thresholds, and ion heating, associated with electrostatic ion-cyclotron waves excited by the inhomogeneous energy-density driven (IEDD) instability in a laboratory experiment are presented. A comparison is made between these waves and the broadband ELF waves recently observed in the auroral ionosphere with sounding rockets and satellites. The measurements of broadband spectra, electron-Landau-resonant phase velocities, low excitation threshold value of parallel electron drift speed, and significant perpendicular ion heating suggest that attributing the broadband ELF waves to the IEDD instability mechanism is justified.

Inhomogeneous transverse electric fields and wave generation in the auroral region: A statistical study

We use data from the Freja satellite to investigate the importance of localized transverse DC electric fields for the generation of broadband waves responsible for ion heating in the auroral region ...

Excitation and propagation of electrostatic ion-cyclotron waves in plasma with structured transverse flow

Attention has been drawn to the recently discovered inhomogeneous energy-density driven (IEDD) instability because of its implications for plasmas in the laboratory and in space. To be excited, the IEDD waves require structured transverse flow, and a range of suitably inhomogeneous transverse-velocity profiles are possible in the West Virginia University Q Machine. This paper describes experimentally measured aspects of this equilibrium essential to the wave growth, the three-dimensional mode structure of the propagating waves, and a transition in the magnetic-field-aligned phase velocity that reveals a new aspect of the instability mechanism.

Inhomogeneous magnetic-field-aligned ion flow measured in a Q machine

Radial profiles of ion flow vd(r) are measured with laser-induced fluorescence for cases in which the flow direction is parallel (vd>0) and/or antiparallel (vd<0) to the equilibrium magnetic field. Experiments are conducted in the barium-ion plasma of a double-ended Q machine. In cases where the ionizers associated with the two ends are not biased relative to each other, two distinct, counterstreaming ion-beam populations are evident. The insertion of blocking electrodes introduces inhomogeneity into the density profiles of the ion populations without effecting the homogeneity of the radial profile of each population’s drift velocity. In cases where the two ionizers are biased relative to each other, a single ion population exists. Variation in the radial profile of the ion population’s parallel drift velocity vd is produced such that (dvd/dr) can be negative or positive with magnitudes 0–70% of the ion gyrofrequency ωci. These results are discussed in the context of beam-driven and velocity-shear-driven ...

Velocity‐shear origin of low‐frequency electrostatic ion‐gyroresonant waves

Electrostatic ion-cyclotron waves with spectral features below the ion gyrofrequency and spaced by small fractions of the ion gyrofrequency are reported, a result not possible for waves associated with the well-known current-driven electrostatic ion-cyclotron (CDEIC) instability. This is accomplished by producing a localized, transverse (to the magnetic field B z^) electric field E(x) x^ in a laboratory plasma to excite inhomogeneous energy-density driven (IEDD) waves and by exploiting the k y ν E dependence of the IEDD mode frequency, where k y and ν E are the components of the IEDD wavevector and plasma drift speed, respectively, along the E × B direction. The IEDD frequency range is shown to reach down to one-third of the ion gyrofrequency, a range usually reserved for other types of waves, e.g., ion-acoustic waves. These results may be relevant to broadband ELF waves observed in the ionosphere.

An effect of neutral collisions on the excitation threshold of electrostatic ion‐cyclotron waves

Laboratory experiments using a sodium plasma are reported that demonstrate a robustness of electrostatic ion-cyclotron waves in the presence of elevated neutral-helium pressure. It is found that the excitation threshold decreases with neutral pressure, and that, in the presence of a transverse localized electric field, the waves are able to tolerate much higher collisionality than previously believed. When the ion-neutral collision frequency and ion gyrofrequency are comparable, the excitation threshold is significantly smaller than its collisionless value.

Ion temperature measurements in helicon plasmas

Summary form only given. Laser induced fluorescence measurements of the ion temperature in a helicon plasma indicate that the perpendicular ion temperature scales linearly with applied magnetic field. Finite gyroradius effects play a negligible role since at the lowest magnetic fields examined, the ion gyroradius remains much smaller than the chamber diameter. In a cylindrical device, Bohm diffusion across the axial magnetic field leads to a diffusion coefficient which depends inversely on the magnetic field strength. Assuming that the diffusion coefficient determines both the particle and energy confinement times, linear scaling of the ion temperature with magnetic field suggests that the ions are heated by collisions with the electrons. Measurements of the parallel and perpendicular ion temperatures and the electron density versus magnetic field in helicon source will be presented. Ongoing attempts to demonstrate ion cyclotron resonant heating in helicon plasmas will also be discussed and the latest results presented.

A laboratory quiescent plasma device for modeling velocity shear in space

Summary form only given. A continuing topic of interest among the space physics community is the acceleration of low-altitude ionospheric ions and the underlying mechanisms. One such mechanism, which has gained much attention in the past few years, is ion acceleration due to turbulence generated by a velocity-shear layer transverse to the local magnetic field, in particular, in situations where magnetic-field-aligned currents may be too small to destabilize current-driven electrostatic ion-cyclotron modes. These situations are modeled in a plasma produced by the WVU Q-machine. The plasma is a collisionless, low-temperature (T/sub i//spl ap/T/sub e/=0.2 eV), low-neutral-density alkali metal, typically sodium or barium, confined by a uniform magnetic field (B=0.5-3.0 kG). The shear layer is created by a segmented-disk electrode placed at the end of the plasma column opposite to the source; the voltage drop across any two groups of segments creates a radially localized electric field, and thus an inhomogeneous azimuthal E/spl times/B plasma drift. A variety of space-relevant parameters can be adjusted, such as the ratio of gyroradius to inhomogeneity scale length (/spl rho//sub i//L), ion-electron temperature ratio (T/sub i//T/sub e/), ratio of ions to neutrals (to model different altitudes), and ratio of ion densities in the case of multi-ion-species plasma studies.

Velocity-shear origin of broadband electrostatic noise

Summary form only given, as flows. Multiple, independent eigenmodes of an inhomogeneity-driven instability are observed simultaneously with large and comparable amplitude. This is in sharp contrast to the usual case in plasmas in which one eigenmode dominates or, for conditions far above the excitation threshold, a collection of harmonics appears. The significance is that the spectral features are not separated by the ion gyrofrequency /spl omega//sub ci/, or any other normal-mode frequency of the plasma. This has implications regarding the spectral width of the fluctuations, which, in some cases, can exceed 50% of center frequency. Furthermore, the spectral features are Doppler downshifted, in some cases to frequencies associated with ion-acoustic modes. For example, /spl omega///spl omega//sub ci/=0.25 are readily observed (recall that T/sub i//T/sub e/ is of order unity). These facts are particularly unexpected because the plasma conditions correspond to large excitation thresholds for current-driven electrostatic ion-cyclotron (CDEIC) and ion-acoustic (IA) waves. The results have direct relevance to recent SCIFER and AMICIST rocket experiments in the Earths cleft ion fountain where electrostatic waves (/spl omega/</spl omega//sub ci/) are observed and are strongly correlated with transverse ion heating and reduced plasma densities, but with the unexplained characteristic that the spectral features are not separated by /spl omega//sub ci/.